write/elf: add Writer::reserve_null_symbol_index (#402)

Use this to ensure `Object` creates the null symbol
when no symbols are defined.

src/write/elf/object.rs

@@ -215,6 +215,7 @@ impl<'a> Object<'a> {
 
         // Calculate index of symbols and add symbol strings to strtab.
         let mut symbol_offsets = vec![SymbolOffsets::default(); self.symbols.len()];
+        writer.reserve_null_symbol_index();
         // Local symbols must come before global.
         for (index, symbol) in self.symbols.iter().enumerate() {
             if symbol.is_local() {

src/write/elf/writer.rs

@@ -699,6 +699,23 @@ impl<'a> Writer<'a> {
         });
     }
 
+    /// Reserve the null symbol table entry.
+    ///
+    /// This will be stored in the `.symtab` section.
+    ///
+    /// The null symbol table entry is usually automatically reserved,
+    /// but this can be used to force an empty symbol table.
+    ///
+    /// This must be called before [`Self::reserve_symtab`].
+    pub fn reserve_null_symbol_index(&mut self) -> SymbolIndex {
+        debug_assert_eq!(self.symtab_offset, 0);
+        debug_assert_eq!(self.symtab_num, 0);
+        self.symtab_num = 1;
+        // The symtab must link to a strtab.
+        self.need_strtab = true;
+        SymbolIndex(0)
+    }
+
     /// Reserve a symbol table entry.
     ///
     /// This will be stored in the `.symtab` section.